To calculate aggregate value, the amount assigned for the category "Less than $10,000" is $9,000. The amount assigned to the category "$1,000,000 or more" is $1,250,000. Aggregate value is rounded to the nearest hundred dollars. (For more information on aggregates, see "Derived Measures".)

Specified owner-occupied and specified vacant-for-sale housing units include only 1-family houses on less than 10 acres without a business or medical office on the property. The data for "specified units" exclude mobile homes, houses with a business or medical office, houses on 10 or more acres, and housing units in multiunit buildings.

The data on mortgage status were obtained from answers to long-form questionnaire Items 47a and 48a, which were asked on a sample basis at owner-occupied housing units. "Mortgage" refers to all forms of debt where the property is pledged as security for repayment of the debt, including deeds of trust; trust deeds; contracts to purchase; land contracts; junior mortgages; and home equity loans.

A mortgage is considered a first mortgage if it has prior claim over any other mortgage or if it is the only mortgage on the property. All other mortgages, (second, third, etc.) are considered junior mortgages. A home equity loan is generally a junior mortgage. If no first mortgage is reported, but a junior mortgage or home equity loan is reported, then the loan is considered a first mortgage.

In most census data products, the tabulations for "Selected Monthly Owner Costs" and "Selected Monthly Owner Costs as a Percentage of Household Income in 1999" usually are shown separately for units "with a mortgage" and for units "not mortgaged." The category "not mortgaged" is comprised of housing units owned free and clear of debt.

A question on mortgage status was included in the 1940 and 1950 censuses, but not in the 1960 and 1970 censuses. The item was reinstated in 1980 along with a separate question dealing with the existence of second or junior mortgages. In 1990, the mortgage status questions were asked of 1-family, owner-occupied housing units; mobile homes; and condominiums. In 1990, the answer categories for the second and junior mortgage question did not distinguish between a second mortgage and a home equity loan.

In Census 2000, the questions were asked at all owner-occupied housing units. In addition, the answer categories distinguished between a second mortgage and a home equity loan.